The present invention relates to a sheet-feeding device for printed sheets in a printing press, and to a method for operating the sheet-feeding device.
It is known for printed sheets of paper at the end of a printing press to be transferred to a sheet-feeding device, in which as a rule the sheets are powdered and then set down to form a stack. To that end, known sheet-feeding devices have revolving chains, on which grippers extending crosswise to the transport direction are secured, with which the front end edge of the sheet is grasped so that this edge can be transported through the sheet-feeding device. Such grippers have the grave disadvantage of having a relatively large structural volume, so that the powdering device must be spaced far enough away from the sheet surface that the grippers can be passed beneath the powdering device. Furthermore, the paper sheets, which are grasped merely on the front end edge, have a tendency to flutter at the usual transport speed of several meters per second in modern printing presses, which can lead to impairment of the printed image. Furthermore especially if the sheet is printed on both sides, the sheet has to be transported on an air cushion.
The object of the present invention is therefore to furnish a sheet-feeding device with which the sheet can be transported with fewer problems from the printing press until it is stacked.
This object is attained according to the present invention with a sheet-feeding device having at least one linear motor for transporting the grippers, with the grippers being secured to a carriage which is part of the linear motor, the carriages being movable on a primary part embodied as a stator.
The great advantage of the present invention is considered to be that because a linear motor is used the individual carriages can be triggered in a targeted way and thus independently of one another. In conventional sheet-feeding devices, the individual grippers are rigidly joined together by way of the revolving chains so that all the grippers have the same transport speed. In the delivery system of the present invention conversely, the carriages and thus the grippers can be triggered in a targeted way and above all are independent of one another in terms of their relative courses of motion. Furthermore, with such linear motors, relatively high speeds and very high accelerations and decelerations can be achieved. A further advantage is considered to be that such linear motors are virtually wear-free and operate with extremely little noise. Finally, by way of triggering the individual carriages, sheets of any arbitrary size, can be grasped.